It is known, from statistics published in 1995, that the number of diabetes patients in the United States is 7.8 million, or about 3.4% of the total U.S. population. This number has been steadily rising over the last 25 years. Approximately 90%, or about 7 million, are non-insulin-dependent diabetes mellitus (NIDDM) patients, in whom the sensitivity to rising glucose levels, or the responsiveness of insulin, is compromised to varying degrees. About 30%, or 2.3 million these patients, use insulin, and about 25% of these insulin users take daily measures of blood glucose levels. As a general proposition, most NIDDM patients are candidates for blood glucose level measurements and/or injections of supplemental insulin. The percentage of NIDDM patients receiving insulin treatment increases with the duration of NIDDM, from an initial rate of about 25% to about 60% after 20 years. For this population of patients, there is a need for a flexible and reliable system and method for measuring glucose level and supplying insulin when and as needed.
The human pancreas normally provides insulin for metabolic control. Basically, the insulin acts to promote transport of glucose in body cells. The pancreas has an endocrine portion which, among other functions, continuously monitors absolute blood glucose values and responds by production of insulin as necessary. The insulin-producing cells are beta cells, which are organized with other endocrine cells in islets of Langerhans; roughly 60-80% of the cells in an islet are such beta cells. The islets of Langerhans in turn are distributed in the pancreatic tissue, with islets varying in size from only about 40 cells to about 5,000 cells.
It has been observed that neighbor beta cells within an islet are coupled by gap junctions, which allow for electrical coupling and communication between neighboring beta cells. The beta cells within the islet undergo periodic depolarization, which is manifested in oscillatory electrical spikes produced by the beta cells, often referred to as burst which carries on for a number of seconds. The beta cell electrical activity is characterized by a low frequency alternation consisting of a depolarized phase (the burst) followed by a repolarized or hyperpolarized phase which is electrically silent. The relative time spent in the depolarized phase, during which the relatively higher frequency beta cell action potentials are triggered, has a sigmoidal relation with blood glucose concentration. The duty cycle, or depolarization portion compared to the quiet portion, is indicative of glucose level, and thus of insulin demand. Additionally, the frequency of the spikes during the active period, and likewise the naturally occurring frequency of the bursts (also referred to plateaus) carries information reflective of glucose level.
In view of the above, it is to be seen that sensing of the beta cell activity from islets of Langerhans in the pancreas may provide information for sensing insulin demand and controlling insulin delivery. Systems which seek to utilize glucose-sensitive living cells, such as beta cells, to monitor blood glucose levels, are known in the art. U.S. Pat. No. 5,190,041 discloses capsules containing glucose-sensitive cells such as pancreatic beta cells, and electrodes for detecting electrical activity. The capsules are situated similarly to endogenous insulin-secreting glucose-sensitive cells, and signals therefrom are detected and interpreted to give a reading representative of blood glucose levels. However, in this and other similar systems, the problem is in reliably sensing the beta cell electrical activity. It is difficult to determine the onset of the burst phase, and accurate determination of the spike frequency is difficult. This sensing problem is aggravated by cardiac electrical interference, as sensing of the QRS can mask portions of the islet electrical activity, particularly the onset of the burst depolarization phase. Thus, there is a need for a system which effectively and reliably utilizes the body's own glucose-monitoring system for obtaining accurate information concerning blood glucose level and insulin demand. Additionally, it is very desirable to provide for an effective response to rising insulin demand by activating an insulin pump, or by enhancing pancreatic insulin production.